The invention disclosed and claimed herein pertains generally to the field of equalizer or equalization apparatus, that is, apparatus which is located at the receiving end of a data transmission channel and which comprises an inverse model thereof. More particularly, the invention pertains to equalizer apparatus which has a very rapid convergence time, i.e., which is capable of being set up as an inverse model of a data transmission channel in a very short time period. Even more particularly, the invention pertains to equalizer apparatus which is useful for employment with a data transmission channel which is subject to fading, i.e., frequent or continual change in channel characteristics.
As is well known in the art, an equalizer is a type of filter, a filter being a device which is capable of providing a desired output in response to a specified input. The input to an equalizer comprises data which has been distorted by passage through a data transmission channel, and the desired equalizer output is the data in its original, undistorted form. Equalizer devices, which were invented in the mid-1960's, may be set up to provide inverse models of data transmission channels with which they are used, as aforementioned, and are located at the receiving ends thereof. When channel distorted data is coupled into an equalizer, it is inversely distorted thereby, so that the equalizer output comprises the data in undistorted condition.
In order to set up, or "train", an equalizer as an inverse transmission channel model, a sequence of prespecified signals, referred to as training signals, are coupled directly to the equalizer, so that they are received thereby without distortion. By determining the error between the undistorted and channel distorted (i.e. transmitted) forms of successive training signals, and by adjusting the equalizer in relation to such errors, the equalizer is adapted to become the aforementioned inverse channel model by the time the training signal sequence concludes.
Presently available equalizers generally comprise configurations of adjustable electrical components, together with some means for varying respective parameters of the components in accordance with successive training signal errors. Such means include calculation or like devices for performing a series of mathematical computations in response to each training signal error. It will be readily apparent that the efficiency of an equalizer device is closely related to the length of the training signal sequence which is required to set the equalizer up, and also to the number and complexity of the computations which are required per training signal error.
Efficiency of operation is very important in an equalizer device for reasons of cost and complexity of fabrication. Efficiency may be even more important as a determinant of equalizer convergence time, which, as aforementioned, is the time that is required to adapt an equalizer to inversely model the characteristics of a particular data transmission channel. Covergence time is especially important where the data transmission channel is subject to continual variations in channel characteristics, and data is transmitted at a very high rate of speed. If an equalizer employed with such channel is incapable of adapting or converging to the new channel characteristics with sufficient speed, significant amounts of transmitted data may be lost.
By means of their invention, Applicants provide significant improvements over the prior art in both the convergence time and operational efficiency of equalizer apparatus. Such apparatus is considered to have particular utility in the operation of a communication system wherein data is transmitted at a rapid rate through a channel which is subject to frequent or continual fading. However, it is not intended to limit the invention to such application.